Professor James Economy's Group
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[Polymer Blends] [Microelectronic Polymers]
[Liquid Crystalline
Polyesters] [Polyester Thermosetting Resins]
Liquid
Crystalline Polyesters Annealing
LC Copolyesters Thermotropic
liquid crystalline (LC) copolyesters have found a wide
range of high tempreature applications such as electrical
connectors, high temperature adhesives, and protective
coatings. Critical to the successful implementation of
the above technologies is the need for a fumdamental
understanding of the microstructure change by high
temperature annealing. Based on our prefvios work we
briefly describe some of our earlier work elucidating the
nature of high temperature annealing in LC copolyesters.
The following discusses three major areas that we feel
will elucidate the nature of LC copolyester
microstructure. Randomization
Process Several
years ago Economy et al. took an initial step to certify
the randomization process of HBA/HNA copolyesters using 13C
NMR. When a mixture of homopolymers was compression
molded at 450°C, within 10 seconds, the NMR spectrum of
the sample displayed four carboxyl resonances which
corresponds to the four dyad arrangements. These results
can be interpreted as the first direct evidence for
randomization by interchain transesterification reactions
(ITR, see Polyester Thermosetting Resins). These
reactions are very rapid; with 1,000 bond swappings
happening every 10 seconds. Because of this rapid
character we have recently shown that homogeneously
strong, high temperature, adhesive bonds can be made
between HBA/HNA copolyester films on metal substrates. Ordering
Process Annealing
LC copolyesters near the transition temperature
dramatically increases the transition temperature via
ordering processes. A change in microstructure from
hexagonal to orthorhombic, an increase in density, and a
decrease in solubility are key indications of an increase
in ordering. Schneggenburger et al. reported on a change
of sequence distribution by ordering processes. When a
random copolyester is annealed near its transition
temperature, 13C NMR results show a shift in
the BN:BB diad peak ratio indicating a change in sequence
distribution by ITR. An
Unusual Degradation Process An
unexpected degradation process was observed in step
ordering of the HBA/HNA copolyesters. The fact that this
degradation process occurs only in HBA/HNA copolyesters
suggest that the naphthalene unit may be more prone to
these kinds of rections. Stepwise annealing of the
HBA/HNA copolyesters resulted in a complete loss of
crystallinity. These degredation reactions occur only
within the highly ordered crystalline orthorhombic
structures. The characteristics of the degradation of
HBA/HNA copolyesters include reduced flow properties,
decreased solubility, discoloration, and thermal
irreversible properties. Currently we are researching the
rheological properties to understand the change of
molecular structure by step order annealing processes. |
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